It is well known that normal clothing fabric when used in environments such as a clean room, hospital operating room or other areas where humidity is reduced or controlled, have the drawback of becoming charged with static electricity as a result of friction. Due to the reduced humidity the static electricity is not readily dissipated into the air. Likewise, when ordinary cloth is used as a filter medium, the friction caused by the air and particles passing through the cloth results in the buildup of static electricity. As a result, dust particles tend to cling to such cloth or filter medium decreasing the permeability of the filter reducing air flow through the filter. In some instances, the buildup of the static electricity can result in discharge of a spark traveling between the cloth and an object in the environment which is either grounded or has the opposite relative static discharge. The spark caused by the rapid discharge of static electricity can cause explosions in areas having explosive atmospheres such as hospital operating rooms or oxygen-rich environments which electronic components may be used, assembled or tested. The rapid discharge of static electricity may also cause dust explosions.
There is a need for electrostatic dissipating fabrics which can be used to construct garments, filter media and drop cloths which have characteristics appropriate for each end use. For example, the fabric used for the production of garments must be comfortable, washable without loss of its static dissipating properties, must be economical, and should be relatively opaque. For filter media, the electrostatic dissipating fabric should have a uniform and controllable permeability to air or other gases passing through the filter. For drop cloth applications, the electrostatic dissipating fabric should be fireproof yet reasonably economical.
In U.S. Pat. No. 2,845,962 an anti-static fabric is disclosed which is made from a fibrous material containing electrically conductive carbon black in combination with fibrous material free from the carbon black. Such fabric, however, does not have the desired conductivity unless a substantial amount of carbon black is used thereby resulting in reduced mechanical strength of the fiber, often leading to the fabric breaking during the processing step. Further, the appearance and hand of the product produced from fiber containing a substantial amount of carbon black is generally unsatisfactory.
U.S. Pat. No. 3,288,175 teaches the incorporation of a small quantity of metallic fibers in the textile fiber materials to produce an anti-static fabric. Such fabric, however, is difficult to process and results in a fabric which has an unsatisfactory hand. Further, a fabric produced in accordance with such method is relatively expensive.
U.S. Pat. No. 3,586,597 teaches the use of a fiber which is coated with a resinous matrix of finely divided silver or carbon black. Such coating, however, is not always sufficient to provide the necessary anti-static properties and often results in loss of anti-static properties as a result of normal processing such as dying, cutting and so forth. If such fabric is washed further degradation of the anti-static properties of the fabric occurs.
Likewise, the anti-static cloth described in U.S. Pat. No. 3,986,530 which is formed from an electrically conductive thread having electroless metal plated staple fibers and metallic filaments combined in a specific amount suffers a degradation of anti-static performance from loss of the coating as the fabric is flexed or otherwise processed or washed.